The present invention relates to a semiconductor laser control circuit and, more particularly, to a semiconductor laser control circuit to be used with an optical information recording/playing system for recording information in an optical disc capable of optically recording and playing it and for playing the recorded information from the optical disc.
In a certain rewrite type optical disc, one optical beam is used to accomplish the recording operation with an optical power at the peak value of a pulse light while erasing the recorded information with the bottom value of the pulse light. The recording and erasing characteristics necessary for the recording and playing operations of such rewrite type optical disc may not be obtained unless the peak and bottom values of the recording pulse light are precisely controlled. The present invention contemplates to provide a semiconductor laser circuit for independently and precisely controlling the peak and bottom values of the pulse light and for setting them at necessary optical power values at high speeds. According to the present invention, a highly reliable optical recording/playing system can be constructed by detecting an abnormality of the output light of a semiconductor laser to prevent the recorded content of the optical disc from being broken.
FIG. 12 is a circuit diagram showing the structure of a semiconductor laser control circuit according to the prior art. A semiconductor laser 1 has its output light received by an optical detector 2, the output current of which is subjected by an operation amplifier 3 to a current-voltage conversion so that it is inputted as a control error signal to a controlling operation amplifier 4. Designated at reference numeral 5 is a switch (SW) for switching such a reference voltage to a recording (R) side or a playing (P) side as is used for setting the level of the output light of the semiconductor laser 1. The operation amplifier 4 compares the reference voltage 6 or 7 and the control error signal to control the output light of the semiconductor laser to a constant level. For recording operations, the output light of the semiconductor laser 1 is subjected to a pulse modulation with a recording signal 8. This recording signal is level-shifted at 9, and the pulse amplitude value of the output light is set by a variable resistor 10. A switch 11 is turned on for the recording operations and is electrostatically coupled to the output voltage of the operation amplifier 4 by a capacitor 12. For the recording operations, therefore, the average level of the pulse-modulated output light provides the control error signal so that a control is accomplished to make the average value constant. The output light of the semiconductor laser 1 is pulse-modulated with a recording signal which is electrostatically coupled around that average value.
However, the structure described above is accompanied by a problem that the peak or bottom value of the pulse-modulated output light highly varies, when the optical-output/current characteristics of the semiconductor laser 1 change due to a temperature change or aging, so that the recording/playing characteristics of the optical disc are degraded. FIG. 13 is a diagram illustrating the optical output waveforms of the prior art for explaining that problem. FIG. 13 illustrates the relationships between the optical-output/current characteristics of the semiconductor laser 1 and the waveforms of the pulse-modulated output light. In FIG. 13, a curve I.sub.0 plots the characteristics at a temperature T.sub.0. In the case of recording the optical disc, it is necessary to accurately control not only the peak value P.sub.R of the pulse-modulated output light but also the bottom value P.sub.B. For example, an optical disc of an unerasable material may be recorded with the optical power at the peak value P.sub.R while being preheated by the optical power at the bottom value P.sub.B. Depending upon the characteristics of the material, those peak value P.sub.R and bottom value .sub.B may have to be precisely controlled. In the case of the optical disc of another erasable material, the recording operations may be accomplished with the optical power at the peak value P.sub.R while erasing with the optical power at the bottom value P.sub.B. In this erasable material, too, the necessary recording and erasing characteristics may not be obtained unless the peak value P.sub.R and bottom value P.sub.B of the optical output waveforms are precisely controlled. Here, let it be assumed that the characteristics are varied I.sub.0 -I.sub.1 when the temperature of the semiconductor laser 1 is varied T.sub.0 -T.sub.1. The recording signal 8 is converted to a constant current amplitude I.sub.P and applied to the semiconductor laser 1 so that it is pulse-modulated around a control average level P.sub.M. As a result, the average value at the temperature T.sub.1 is so controlled that the average level P.sub.M may be constant even if the characteristics are varied to I.sub.1. Since, however, the current amplitude I.sub.P of the recording signal is constant even if the curve of the optical-output/current characteristics has its gradient varied as I.sub.1, the peak and bottom values of the pulse-modulated optical output are highly varied to P.sub.R ' and P.sub.B ', respectively. Here have been described the variations of the optical output resulting from the change T.sub.0 -T.sub.1 of the temperature characteristics. Similar degradations in the characteristics take place as I.sub.0 -I.sub.1, too, in the aging of the semiconductor laser 1.
As has been described above, as shown in FIG. 12, the structure of the prior art is troubled by a problem that the peak and bottom values of the pulse-modulated optical output are highly varied due to the temperature characteristics and aging of the semiconductor laser 1. Moreover, the optical power of the pulse-modulated optical output waveforms having such peak and bottom values cannot be measured other than its average value P.sub.M by the ordinary optical power meter, but the peak value P.sub.R and the bottom value P.sub.B have to be set by observing the waveforms. The optical power setting through those waveform measurement finds it difficult to accomplish the precise measurements. If, moreover, the current amplitude value I.sub.P is varied by the variable resistor 10, both the peak value P.sub.R and the bottom value P.sub.B are so varied that they are difficult to set independently of each other. Thus, the prior art is accompanied by the second drawback that the optical power is difficult to set.